CLASSIFICATION OF LOCATIONS FOR ELECTRICAL INSTALLATIONS IN GAS UTILITY AREAS
Gerry Pickens
Atmos Energy
810 Crescent Centre Drive
Franklin, TN 37067
The National Electrical Code (NEC) defines
hazardous locations as those areas "where fire or
explosion hazards may exist due to flammable gases
or vapors, flammable liquids, combustible dust, or
ignitable fibers or flyings."
A substantial part of the NEC is devoted to the
discussion of hazardous locations. That's because
electrical equipment can become a source of ignition
in these volatile areas. Articles 500 through 504, and
510 through 517 provide classification and
installation standards for the use of electrical
equipment in these locations. The writers of the NEC
developed a short-hand method of describing areas
classified as hazardous locations. One of the purposes
of this discussion is to explain this classification
system. Hazardous locations are classified in three
ways by the National Electrical Code: TYPE,
CONDITION, and NATURE.
Class II Locations
The second type of hazard listed by the National
Electrical Code are those areas made hazardous by
the presence of combustible dust. These are referred
to in the Code as "Class II Locations." Finely
pulverized material, suspended in the atmosphere,
can cause as powerful an explosion as one occurring
at a petroleum refinery. Some typical Class II
locations are:
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Hazardous Location Types
Class I Locations
According to the NEC, there are three types of
hazardous locations. The first type of hazard is one
which is created by the presence of flammable gases
or vapors in the air, such as natural gas or gasoline
vapor. When these materials are found in the
atmosphere, a potential for explosion exists, which
could be ignited if an electrical or other source of
ignition is present. The Code writers have referred to
this first type of hazard as Class I. So, a Class I
Hazardous Location is one in which flammable
gases or vapors may be present in the air in sufficient
quantities to be explosive or ignitable. Some typical
Class I locations are:
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Petroleum refineries, and gasoline storage
and dispensing areas;
Dry cleaning plants where vapors from
cleaning fluids can be present;
Spray finishing areas;
Aircraft hangars and fuel servicing areas;
and
Utility gas plants, and operations involving
storage and handling of liquefied petroleum
gas or natural gas.
All of these are Class I . . . gas or vapor . . .
hazardous locations. All require special
Class I hazardous location equipment.
Grain elevators;
Flour and feed mills;
Plants that manufacture, use or store
magnesium or aluminum powders;
Producers of plastics, medicines and
fireworks;
Producers of starch or candies;
Spice-grinding plants, sugar plants and
cocoa plants; and
Coal preparation plants and other carbon
handling or processing areas.
Class III Locations
Class III hazardous locations, according to the NEC,
are areas where there are easily-ignitable fibers or
flyings present, due to the types of materials being
handled, stored, or processed. The fibers and flyings
are not likely to be suspended in the air, but can
collect around machinery or on lighting fixtures and
where heat, a spark or hot metal can ignite them.
Some typical Class III locations are:
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Textile mills, cotton gins;
Cotton seed mills, flax processing plants;
and
Plants that shape, pulverize or cut wood and
create sawdust or flyings.
Hazardous Location Conditions
In addition to the types of hazardous locations,
the National Electrical Code also concerns itself
with the kinds of conditions under which these
hazards are present. The Code specifies that
hazardous material may exist in several different
kinds of conditions which, for simplicity, can be
described as, first, normal conditions, and,
second, abnormal conditions.
In the normal condition, the hazard would be
expected to be present in everyday production
operations or during frequent repair and maintenance
activity.
When the hazardous material is expected to be
confined within closed containers or closed systems
and will be present only through accidental rupture,
breakage or unusual faulty operation, the situation
could be called "abnormal”.
The Code writers have designated these two kinds of
conditions very simply, as Division 1 - normal and
Division 2 - abnormal. Class I, Class II and Class III
hazardous locations can be either Division 1 or
Division 2.
Good examples of Class I, Division 1 locations
would be the areas near open dome loading facilities
or adjacent to relief valves in a natural gas station,
because the hazardous material would be present
during normal plant operations.
Closed storage drums containing flammable liquids
in an inside storage room would not normally allow
the hazardous vapors to escape into the atmosphere.
But, what happens if one of the containers is leaking?
You've got a Division 2 -abnormal - condition . . . a
Class I, Division 2 hazardous location.
So far we've covered the three types of hazardous
locations:
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Class I - gas or vapor
Class II - dust, and
Class III - fibers and flyings.
The only substance in Group A is acetylene.
Acetylene makes up only a very small percentage of
hazardous locations. Consequently, little equipment
is available for this type of location. Acetylene is a
gas with extremely high explosion pressures.
Group B is another relatively small segment of
classified areas. This group includes hydrogen and
other materials with similar characteristics. If you
follow certain specific restrictions in the Code, some
of these Group B locations, other than hydrogen, can
actually be satisfied with Group C and Group D
equipment.
Group C and Group D are by far the most usual
Class I groups. They comprise the greatest
percentage of all Class I hazardous locations. Found
in Group D are many of the most common flammable
substances such as butane, gasoline, natural gas and
propane.
In Class II - dust locations - we find the hazardous
materials in Groups E, F, and G. These groups are
classified according to the ignition temperature and
the conductivity of the hazardous substance.
Conductivity is an important consideration in Class II
locations, especially with metal dusts.
Metal dusts are categorized in the Code as Group E.
Included here are aluminum and magnesium dusts
and other metal dusts of similar nature.
Group F atmospheres contain such materials as
carbon black, charcoal dust, coal and coke dust.
In Group G we have grain dusts, flour, starch, cocoa,
and similar types of materials.
And secondly, kinds of conditions:
Review
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Division 1 - normal conditions, and
Division 2 - abnormal conditions.
Now let's move on to a discussion of the nature of
hazardous substances.
Nature of Hazardous Substances
The gases and vapors of Class I locations are broken
into four groups by the Code: A, B, C, and D. These
materials are grouped according to the ignition
temperature of the substance, its explosion pressure,
and other flammable characteristics.
Let's quickly review. Hazardous locations are
classified in three ways by the National Electrical
Code: TYPE, CONDITION, and NATURE.
There are three types of hazardous conditions: Class I
- gas and vapor, Class II - dust, and Class III - fibers
and flyings.
There are two kinds of hazardous conditions:
Division 1 - normal, and Division 2 - abnormal.
And finally, there is the nature of the hazardous
substance . . . where we find Groups A, B, C, and D
in Class I locations, and, in Class II locations: Groups
E, F, and G
Hazardous Location Equipment
Sources of Ignition
Let's illustrate our Code "translation" with an
example. How would we classify a storage area
where LP gas is contained in closed tanks? LP gas is
a Class I substance (gas or vapor). It's Division 2
because it would only be in the atmosphere if an
accidental rupture or leakage occurred, and it is
Group D material.
Natural Gas facilities would be classified as Class I,
Division I or II, Group D
The table below summarizes the various hazardous (classified)
locations
Now that we've completed our Code translation,
we're ready to move to the next part of our discussion
- hazardous location equipment. To do this, let's first
take a look at the ways in which electrical equipment
can become a source of ignition. There are three of
them.
Arcs and sparks produced by the normal operation of
equipment, like motor starters, contactors, and
switches, can ignite a hazardous location atmosphere.
The high temperatures of some heat-producing
equipment, such as lamps and lighting fixtures, can
ignite flammable atmospheres if they exceed the
ignition temperature of the hazardous material. The
National Electrical Code requires special marking of
heat - producing equipment with temperatures above
100oC (212oF).
Electrical equipment failure is another way an
explosion could be set off. A burn out of a lamp
socket or shorting of a terminal could spark a real
disaster in a hazardous location.
*NOTE: Electrically conductive dusts are dusts with
a resistivity less than 105 ohm-centimeter.
Classification of Locations In Gas Utility Areas
The best resource for classifying natural gas utility
areas would be AGA Catalog # XL1001
“Classification of Locations for Electrical
Installations in Gas Utility Areas”. This document
discusses the classification of areas based on the
characteristics of the flammable gas, dispersion,
normal and abnormal conditions, ventilation, air
currents and many other factors. Of special interest
are the recommended distances given in Appendix C.
An example is shown here
Equipment Design and Construction
Now let's get down to specific hardware and how it is
designed and constructed to be suitable for hazardous
locations, specifically, with those designed for Class I
gas or vapor applications.
The first requirement for a Class I enclosure is
strength. The enclosure must be strong enough to
contain an explosion within. The walls must be thick
enough to withstand the internal strain. It has to be
explosion-proof in case gas or vapors get inside.
Secondly, it must function at a temperature below the
ignition temperature of the surrounding atmosphere.
The equipment must also provide a way for the
burning gases to escape from the device as they
expand during an internal explosion; but, only after
they have been cooled off and their flames
"quenched." This escape route for the exploding
gases is provided through several types of flame
paths.
One type is the ground surface flame path. Here the
surfaces are ground, mated, and held to a tolerance of
15 ten-thousandths of an inch. This permits gases to
escape, but only after they've been sufficiently
cooled, so they won't ignite the volatile surrounding
atmosphere.
Another kind of flame path is the threaded flame
path. After an explosion, the gas travels out the
threaded joint . . . but as it does, it cools off.
Exploded gases may also escape around the shafts of
operators used in the enclosure. But, here again, close
tolerances are used to quench the burning gas.
Examples of two flame paths are shown below.
need for heavy explosion-containing construction, or
flame paths. This difference explains why Class I,
Division 1 equipment can be called explosion-proof,
and Class II equipment is called dust-ignition proof.
Class II equipment has a different set of
requirements:
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It must seal out the dust.
It must operate below the ignition
temperature of the hazardous substance.
It must allow for a dust blanket. That is, the
build-up of dust collecting on top of the
device that can cause it to run "hot" and
ignite the surrounding atmosphere.
For Class III equipment, there is very little difference
in the design from Class II. Class III equipment must
minimize entrance of fibers and flyings; prevent the
escape of sparks, burning material or hot metal
particles resulting from failure of equipment; and
operate at a temperature that will prevent the ignition
of fibers accumulated on the equipment.
There are many enclosures, devices, and fixtures
suitable for all three classes. This simply means that
it meets the specifications for each individual type. A
Class I device which could contain an explosion of a
specified gas would also have to prevent dust from
entering the enclosure to be suitable for Class II. The
close tolerance of the flame path which cools the
burning gases is also close enough to exclude
explosive dust so that a gasket would not be needed.
You can see how important it is to make certain that
all flame paths are protected during installation and
maintenance, and even during handling, shipping,
and storage of explosion-proof material. Even slight
damage to a flame path can permit burning gases to
escape, igniting the surrounding atmosphere. Also, all
cover bolts must be installed for the same reason. A
single missing bolt could allow the release of flaming
gases.
In designing equipment for Class I, Division 1
locations, it is assumed that the hazardous gases or
vapors will be present and eventually seep into the
enclosure, so there is a very real chance for an
internal explosion to occur.
In the case of Class II, however, the assumptions are
different and so the design is different. In Class II,
the explosive dust is kept away from equipment
housed within the enclosure so that no internal
explosion can take place and there is no longer any
Proper installation of hazardous location equipment
calls for the use of seals. Special fittings are required
to keep hot gases from traveling through the conduit
system igniting other areas if an internal explosion
occurs in a Class I device. They are also needed in
certain situations to keep flammable dusts from
entering dust-ignition-proof enclosures through the
conduit. As shown in the figure below, when arcs and
sparks cause ignition of flammable gases and vapors,
the equipment contains the explosion and vents only
cool gases into the surrounding hazardous area.
pressure - FOUR TIMES the maximum anticipated
pressure of 250 psi.
Wiring Methods
Class I, Division I
Wiring methods in Class I, Division I areas must
comply with the following:
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Sealing fittings are designed to be filled with a
chemical compound after the wires have been pulled.
As the compound hardens, it seals passageways for
dusts and gases. As
shown in the figure
above, in each conduit
run entering an
enclosure for switches,
circuit breakers, fuses,
relays, resistors, or
other apparatus which
may produce arcs,
sparks, or high
temperatures within
Class I locations, conduit seals shall be placed as
close as practicable and in no case more than 18
inches (457 mm) from such enclosures. Again,
consult the Code for specific rules for the use of
seals.
Rigorous standards for hazardous location equipment
have been set. Nationally Recognized Testing
Laboratories conduct actual explosion tests under
laboratory conditions. For each Class I enclosure they
experiment with different mixtures of gas and air . . .
from very lean mixtures (a small percentage of gas)
to very rich mixtures (a high percentage of gas) until
they find the one that creates the greatest explosion
pressure. To pass inspection, the equipment must not
only prevent the ignition of the surrounding
atmosphere, but also be able to withstand a
hydrostatic test where oil is pumped into the
enclosure at high pressure to test the limits of its
strength. The device will not pass unless it can resist
rupture at four times the maximum pressure found in
the explosion tests. For example, if explosion testing
shows a maximum pressure for a junction box of 250
pounds per square inch (psi), to get approval, the box
must be able to withstand 1,000 psi of hydrostatic
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Threaded rigid metal conduit
Threaded steel intermediate metal conduit
Type MI cable with termination fittings
approved for the location
All boxes, fittings, and joints must be
threaded for connection to conduit or cable
terminations and must be explosion proof.
Nonmetallic conduit is permitted when
encased in concrete a minimum of 2 inches
thick and buried 2 feet. Nonmetallic conduit
must transition to metal conduit 2 feet
before emergence.
Class I, Division II
Wiring methods in Class I, Division II areas must
comply with the following:
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Threaded rigid metal conduit
Threaded steel intermediate metal conduit
Enclosed gasketed busways
Enclosed gasketed wireways
Type PLTC cable
Type ITC cable Type MI, MC, MV or TC
cable
Boxes, fittings, and joints are not required to
be explosion proof.
Intrinsically Safe Systems
The theory behind intrinsic safety is to ensure that the
available electrical and thermal energy in the system
is always low enough that ignition of the hazardous
atmosphere cannot occur. This is achieved by
ensuring that only low voltages and currents enter the
hazardous area, and that all electric supply and signal
wires are protected by Zener safety barriers.
Sometimes an alternative type of barrier known as a
galvanic isolation barrier may be used.
In normal uses, electrical equipment often creates
internal tiny sparks in switches, motor brushes,
connectors, and in other places. Such sparks can
ignite flammable substances present in air. A device
termed intrinsically safe is designed to not contain
any components that produce sparks or which can
hold enough energy to produce a spark of sufficient
energy to cause an ignition.
cannot be installed in the same conduit as
nonintrinsically safe systems.
Summary
Regardless of the cause of a hazardous location, it is
necessary that every precaution be taken to guard
against ignition of the atmosphere. Electrical
equipment can be a potential source of ignition
through one of three ways:
Most applications require a signal to be sent out of or
into the hazardous area. The equipment mounted in
the hazardous area must first be approved for use in
an intrinsically safe system. The barriers designed to
protect the system must be mounted outside of the
hazardous area in an area designated as Nonhazardous or Safe in which the hazard is not and will
not be present.
Equipment which has been designed for and is
available for use in hazardous areas with intrinsically
safe barriers includes:
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4-20 mAdc Two Wire Transmitters
Thermocouples
RTDs
Strain Gages
Pressure, Flow, & Level Switches
I/P Converters
Solenoid Valves
Proximity Switches
Infrared Temperature Sensors
Potentiometers
LED Indicating Lights
Magnetic Pickup Flowmeters
Most of the apparatus that is mounted in the
Hazardous area will have to be approved and
certified for use in the Hazardous area with an
approved barrier designed for use with that apparatus.
Some simple devices like thermocouples, RTDs,
LEDs and contacts can be used in the hazardous area
without certification as long as it is wired in
conjunction with an approved barrier.
Intrinsically safe equipment and wiring can be
installed using any of the wiring methods suitable for
unclassified locations. Conduit for intrinsically safe
systems must be sealed as we discussed above and
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Arcs and sparks
High temperatures
Electrical equipment failure
Hazardous location equipment is designed and
constructed to eliminate the potential for ignition of
the atmosphere.
The National Electrical Code is the "Bible" of the
Electrical Industry, and the primary source of
reference for hazardous locations. The NEC is also
the basis for OSHA standard 1926.407, Hazardous
(Classified) Locations. There are several OSHA
standards that require the installation of electrical
wiring and equipment in hazardous (classified)
locations according to the requirements of Subpart K,
Electrical. The NEC should be consulted as a
supplement to the OSHA standards for additional
background information concerning hazardous
locations.
Recommended reading for this topic:
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National Electrical Code, NFPA 70, Chapter
5, Article 500
29 CFR 1910 Subpart S, Electrical 1910.307
NFPA 497, "Classification of Gases,
Vapors, and Dusts for Electrical Equipment
in Hazardous Classified Locations"
NFPA Handbook, "Electrical Installations in
Hazardous Locations, " by P. J. Schram and
M. W. Earley
NFPA 70E, Chapter 5, "Hazardous
(Classified) Locations"
NFPA 325, "Fire Hazard Properties of
Flammable Liquids, Gases, and Volatile
Solids"
ANSI/UL 913, "Intrinsically Safe
Apparatus" NFPA 496, "Purged and
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Pressurized Enclosure for Electrical
Equipment in Hazardous Locations."
American Gas Association Catalog No.
XL1001 “Classification Of Locations For
Electrical Installations In Gas Utility
Areas”, revised by Distribution And
Transmission Engineering Committee,
revised Sept. 2010